The present invention relates to a solid electrolytic capacitor for a variety of electronic equipment and manufacturing methods thereof.
A recent trend of digitalization in electronic equipment has caused increasing demand for downsized capacitors with large capacitances and low impedances at high frequency. In conventional art, the following capacitors are used for high frequency applications: plastic film capacitors; mica capacitors; and multi-layered ceramic capacitors. Also, there are aluminum dry electrolytic capacitors, aluminum solid electrolytic capacitors, tantalum solid electrolytic capacitors and the like. An aluminum dry electrolytic capacitor consists essentially of an etched aluminum anode and a cathode rolled up into a cylinder with a separator in between to form a device, impregnating the separator with a liquid electrolyte.
An aluminum solid electrolytic capacitor or a tantalum solid electrolytic capacitor employs a solid electrolyte to improve performance of the aluminum dry electrolytic capacitor.
A method of manufacturing the solid electrolyte is described: an anode is dipped in manganese nitrate solution; the anode is pyrolytically decomposed in a furnace at approximately 250° C. to 350° C.; a manganese oxide layer as the solid electrolyte is formed on the anode. Capacitors having the solid electrolyte show better frequency characteristics and temperature characteristics compared with capacitors with the liquid electrolyte, as the solid electrolyte is free from problems inherent in liquid electrolytes such as outflows of electrolytes at a high temperature, decrease in capacitance owing to a dry up or function degradations owing to hardening of electrolytes at low temperatures.
Japanese Patent Laid-Open Application No. H02-130906 discloses a solid electrolytic capacitor having a solid electrolyte composed of conductive polymers polymerized from monomers such as pyrrole and thiophene.
A method of manufacturing a solid electrolyte composed of conductive polymers for use in the solid electrolytic capacitor is described below.
First, a pre coat layer composed of conductive materials such as manganese oxide, conductive polymer or the like is formed on a surface of a dielectric oxide film of a valve metal. Next, the solid electrolyte composed of conductive polymers is formed through: (1) an electrolytic polymerization by supplied electricity to the pre coat layer in electrolytic solution for polymerization containing monomer and a dopant, or (2) a chemical polymerization by dipping the pre coat layer in electrolytic solution for polymerization containing an oxidizing agent. Generally, sulfonic acid compounds or phosphoric acid compounds are used as the dopants to produce the solid electrolytic capacitor by the manufacturing methods. The strongly acidic dopants cited above, however, have a problem that undoping of the dopants, especially in highly humid environments, causes damage on the dielectric oxide film of valve metal, or decreases in product reliability due to poor forming. A measure proposed for the problem is to use alkyl aromatic sulfone compound or the like as a dopant to prevent degradation of equivalent series resistances (ESR) and capacitances caused by undoping. Another problem, however, arises that a usable rate of capacitance decreases in this case. The decrease in the usable rate of capacitance is supposedly caused by the facts that:
(1) a polymer is apt to be bulky when dopants having a large steric hindrance such as alkyl aromatic sulfone compound or the like are adopted, and
(2) a polymer is apt to be formed concentrated on edges of rough surfaces of the dielectric oxide film of valve metal.
On the other hand, although an excellent usable rate of capacitance is obtained when a large quantity of compounds with a small steric hindrance such as sulfuric acid or the like are used as the dopants, a significant degradation will occur in characteristics due to undoping of the sulfuric acid under hot and humid environments.